The present invention relates to an image forming apparatus such as a copying machine, a printer or the like using a electrophotographic process or electrostatic recording process.
Recently, a magnetic brush charging member having a magnetic brush portion which is magnetically confined on a carrying member (magnetic charging brush) is considered for use with a photosensitive member to which the magnetic brush is contacted to the photosensitive member to electrically charged it.
In the magnetic brush charger, electroconductive magnetic particles are confined directly to a magnet or combined on a sleeve containing a magnet therein. The magnetic brush is stationary or rotating and contacted to the photosensitive member, and the magnetic brush is supplied with a voltage by which an electric charge is injected into the photosensitive member to start charging.
In the injection charging system, the charge is directly injected into the photosensitive member from the contact charge member by which the surface of the photosensitive member is electrically charged. More particularly, a contact charge member having an intermediate resistance is contacted to the surface of the photosensitive member so that charge injection is effected directly into the surface of the photosensitive member. Therefore, even if the applied voltage to the contact charge member is lower than the discharge threshold, the photosensitive member can be charged to a potential equivalent to the applied voltage. The injection charging system does not produce ions.
However, the state of contact between the contact charge member and the photosensitive member is significantly influential to the charging performance. The contact charge member is desirably constituted at a high density, and the speed difference relative to the photosensitive member is desirably large so as to provide higher opportunity of the contact to the photosensitive member. In this sense, a magnetic brush charger is desirable as a charge member from the standpoint of the stability.
The injection charging using the magnetic brush charger is considered as being equivalent to a series circuit including a resistance and a capacitor. In an ideal charging process, the capacitor is charged at a point on the surface of the photosensitive member during the period of time in which the point is contacted to the magnetic brush (charging nipxc3x97peripheral speed) so that surface potential of the photosensitive member at the point becomes substantially the same as the applied voltage.
The injection charging type is less dependent on the ambience, and does not use electric discharge, and therefore, the applied voltage to the contact charge member is substantially as small as the required potential of the photosensitive member, and an additional advantage is that ozone is not produced, thus accomplishing completely ozoneless charging of low energy consumption. In an image forming apparatus of the contact charging type wherein the use is made with a magnetic brush charger comprising a magnetic particle carrying member for carrying magnetic particles and confining the particles thereon, and the magnetic brush portion of the magnetic brush charger is contacted to the photosensitive member (image bearing member), and wherein a charging bias is applied to electrically charge the photosensitive member, there is a problem of a deposition of the magnetic particles (charging carrier) constituting the magnetic brush of the magnetic brush charger (carrier deposition).
Referring to FIG. 5, the description will be made as to this problem. In the Figure, designated by 1 is a photosensitive drum as an image bearing member, 2A is a magnetic brush charger, 2b is an electroconductive charging sleeve as a magnetic particle carrying member in the magnetic brush charger, 2c is a magnetic brush portion of charging carrier 2d applied on the charging sleeve.
In the charging sleeve 2b, there is provided a magnet roller (unshown) functioning as a magnetic field generating member, and the magnet roller magnetically confines the charging carrier in the form of magnetic particles on the outer circumstantial surface of the charging sleeve 2b by the magnetic force of the magnet roller, so as to form a magnetic brush portion 2c of the charging carrier.
The magnetic brush portion 2c of the magnetic brush charger 2A is contacted to the surface of the photosensitive drum 1 to form a charging nip N.
In the magnetic brush charger 2A, the magnet roller in the charging sleeve 2b is not rotatable, and the charging sleeve 2b around the magnet roller is rotated at a predetermined peripheral speed in a predetermined direction. The photosensitive drum 1 is rotated in a predetermined direction at a predetermined peripheral speed. By the rotations of the charging sleeve 2b and the photosensitive drum 1, the outer surface of the photosensitive drum 1 is uniformly rubbed at the charging nip portion by the magnetic brush portion 2c of the magnetic brush charger 2A. The charging sleeve 2b receives a predetermined charging bias voltage from a charging bias applying voltage source not shown. By this, the outer surface of the photosensitive drum 1 which is rotating is charged uniformly to predetermined potential of a predetermined polarity. Designated by X is a region of a charging sleeve 2b which is coated with the charging carrier (a charging sleeve region in which the magnetic brush portion 2c exists); and Y is a region which is end sides of the charging sleeve of the region X and which is not coated with the charging carrier (a charging sleeve region in which the magnetic brush portion 2c does not exist).
At the boundary portion between the region X and the region Y, the potential of the photosensitive member abruptly changes, and therefore, the charging carrier deposition (end deposition of the charging carrier) from the magnetic brush charger 2A to the surface of the photosensitive drum 1 occurs at the boundary portion due to the potential difference. A method of avoiding this problem is disclosed in Japanese Laid-open Patent Application No. HEI 8-106201. As shown schematically in FIG. 5(b), the surface of the charging sleeve adjacent the end of the coated region is subjected to an insulating treatment 2f (a member for electrically insulating the charging carrier 2d from the developing sleeve 2b) so that change of the potential of the photosensitive member is less steep at the boundary portion, by which the carrier deposition can be prevented.
However, when the insulating 2f is broken or when the lateral current due to electric resistance increase of the charging carrier decreases, the charging carrier deposition is likely to occur again.
In the case that charging carrier 2d deposited on the photosensitive drum 1 is introduced into the developing device, the property of the developer is changed with the result that image forming operation becomes instable. Even when the developer is a two component type developer, the developing carrier has an electric resistance which is higher than the charging carrier, and therefore, the carrier for charging should be different from the carrier for development. If the charging carrier falls to the transferring device, the transfer bias voltage a leak with the result of transfer defect since the charging carrier has an electric resistance which is lower than the developer.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which a possible damage of a developing device by charging particles is avoided.
It is another object of the present invention to provide an image forming apparatus in which a possible damage of a image transfer device by charging particles is avoided.
According to an aspect of the present invention, there is provided an image forming apparatus including an image bearing member; a charging means for electrically charging the image bearing member, the charging means including a particle carrying member for receiving a charging bias voltage and for carrying electroconductive magnetic particles, the particle carrying member being provided with an electrically insulated portion at an end portion within a region in which the magnetic particles are carried; electrostatic image forming means for forming an electrostatic image on the image bearing member charged by the charging means; and developing means for developing the electrostatic image on the image bearing member, the developing means including a developer carrying member for carrying a magnetic developer and a magnetic field generating means provided in the developer carrying member; wherein the electrically insulating portion is outside a longitudinal end of the magnetic field generating means.
These and other objects, features and advantage of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.